Pd nanoparticles immobilized on Fe3O4 @ poly(ethylene glycol) bridged amine functionalized imidazolium ionic liquid: A magnetically separable catalyst for heck in water

Article abstract of DOI:10.1007/s10562-015-1534-3

A palladium-based catalyst supported on novel Fe₃O₄ @ Poly(ethylene glycol) bridged amine functionalized imidazolium ionic liquid (Fe₃O₄@PEG-AIm-NH₂- IL) was successfully prepared by a facile method. The catalyst was characterized by FT-IR, TEM, VSM, XRD, XPS and TGA-DSC, which showed high activity and stability for the Heck reaction in pure water. Furthermore, the catalyst could be recovered in a facile manner from the reaction mixture and recycled five times without any loss in activity.

Full text of DOI:10.1007/s10562-015-1534-3

Catal Lett
DOI 10.1007/s10562-015-1534-3
Pd Nanoparticles Immobilized on Fe3O4 @ Poly(ethylene glycol)
Bridged Amine Functionalized Imidazolium Ionic Liquid:
A Magnetically Separable Catalyst for Heck in Water
Xiang Liu1 Xiaohua Zhao2 Ming Lu1
•
•
Received: 10 February 2015 / Accepted: 12 April 2015
Ó Springer Science+Business Media New York 2015
Abstract A palladium-based catalyst supported on novel
Fe3O4 @ Poly(ethylene glycol) bridged amine functional-
ized imidazolium ionic liquid (Fe3O4@PEG–AIm–NH2–
IL) was successfully prepared by a facile method. The
catalyst was characterized by FT-IR, TEM, VSM, XRD,
XPS and TGA-DSC, which showed high activity and sta-
bility for the Heck reaction in pure water. Furthermore, the
catalyst could be recovered in a facile manner from the
reaction mixture and recycled five times without any loss in
activity.
Graphical Abstract A novel magnetically separable Fe3
O4@PEG–AIm–NH2–IL–Pd was designed and synthe-
sized, which was used as outstanding palladium catalyst for
the Heck reaction in pure water. The supported catalyst
could be recycled and reused with the simple application of
an external magnetic field, while keeping similar catalytic
activity for five successive reactions.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-015-1534-3) contains supplementary
material, which is available to authorized users.
& Xiaohua Zhao
zhao12_19@163.com
& Ming Lu
luming302@126.com
1
School of Chemical Engineering, Nanjing University of
Science and Technology, Nanjing 210094, China
2
School of Materials Science and Engineering, Jiangsu
University, Zhenjiang 212013, China
123

X. Liu et al.
Keywords Fe3O4@PEG–AIm–NH2–IL–Pd Á Heck
reaction Á Pure water
nanoparticles. On the other hand, PEG-supported ILs-
palladium complexes exhibited superior water-dispersible
character, and made the organic reaction occur in green
water. Liu and co-workers synthesized three PEG-func-
tionalized imidazolium salts which served as N-hetero-
cyclic carbine precursors for the palladium-catalyzed
Suzuki-Miyaura reaction in water. However, those cata-
lysts could not be recovered [21].
1 Introduction
Ionic liquids (ILs) have attracted considerable interest
during the past decade due to their unique properties [1–3].
Nevertheless, their widespread application in catalytic
processes is still restricted by several practical problems.
One main disadvantage is the use of a large amount of ILs
as solvents, which are expensive despite the fact that many
ILs are commercially available today. In addition, tradi-
tional ILs show remarkable solubility in common solvents,
which causes inconvenience of product separation and
catalyst recover [4]. Considering this, immobilization
strategies for ILs have been developed and open up new
possibilities because the new materials retain important
physical and chemical properties of ILs and feature facile
catalyst recycling and reuse [5]. This new class of materials
combines the advantages of ILs with those of support
materials, while overcome the main drawbacks of tradi-
tional ILs. In recent years, a number of supported ILs
materials have been designed, prepared and used in syn-
thetic chemistry [6–11], in particular polymer based ionic
liquids [12–16]. Among polymer supported ILs, poly
(ethylene glycol)s (PEGs) bridged imidazolium-based ILs
have been developed and used successfully as recycled
solvents for various organic reactions [17–20]. On the one
hand, imidazolium-based ILs are able to stabilize metal
Compared with the conventional separation, magnetic
separation has emerged as a robust, highly efficient and fast
separation tool for product and catalyst. The catalyst im-
mobilized on superpara magnetic nanomaterials can be
easily separated from the reaction medium using an ex-
ternal permanent magnet, which provides a simple
separation of the catalyst without the need for filtration,
centrifugation or other tedious workup processes [22]. In
addition, an interesting feature of magnetic nanoparticles is
their convenient surface modification that provides a wide
range of magnetic-functionalized catalysts showing iden-
tical and sometimes even higher activity than their homo-
geneous counterparts in organic transformations. More
recently, functionalized magnetite Fe3O4 nanoparticles
have been emerged widely for various organic reactions
including C–C couplings [23–27].
To the best of our knowledge, there are only a few
reports on the application of magnetically separable Pd-
catalyzed Heck reaction in neat water. Herein, we report a
novel catalyst composed of a Fe3O4 core and a shell con-
sisting of Pd nanoparticles stabilized by PEG bridged
amine functionalized imidazolium IL (Fe3O4@PEG–AIm–
123

Pd Nanoparticles Immobilized on Fe3O4 @ Poly(ethylene glycol) Bridged Amine Functionalized…
NH2–IL–Pd), which makes the catalyst magnetically re-
coverable and high water-dispersible. This is the first ex-
ample on the application for the Heck reaction in pure
water using magnetically separable PEG-functionalized ILs
palladium complex as catalyst.
reducing Pd2? to Pd0 with hydrazine. The solution was
removed and the resulting solid (Fe3O4@PEG–AIm–IL–
Pd) was recovered magnetically, washed three times with
deionized water and dried at room temperature. The con-
tent of Pd was estimated to be 0.105 mmol g-1 based on
ICP.
2.4 General Conditions for Heck Reactions
2 Experimental
Aryl halide (1.0 mmol), alkene (2.0 mmol), base (2.0 m-
mol), and 0.05 % mol of palladium were added to the
vessel with 3 mL water. The mixture was continuously
stirred at 80 °C for the desired time until complete con-
sumption of the starting aryl halide. The reaction progress
was monitored by GC. After magnetic separation of the
catalyst, the product was extracted with diethyl ether and
purified by column chromatography.
2.1 General Information
The PEG4000 bridged amine functionalized imidazolium ionic
liquid and bare Fe3O4 NPs were prepared by the procedure
given in the supporting material. Other chemicals and solvents
were purchased from commercial suppliers. NMR spectra were
recorded on Bruker DRX300 spectrometer. IR spectra were
recorded on NICOLET NEXUS870. Products were identified
using a 6,820 gas chromatograph (GC) with an Agilent Tech-
nologies HP-Innowax (30 m 9 0.32 mm 9 0.5 lm). Mass
spectroscopy data were collected on an MS-EI instrument.
Palladium content of the catalyst was measured by inductively
coupled plasma (ICP) on PE5300DV analyzer. XRD data were
collectedwithCuKa radiationonBruker D8ADVANCE. TEM
images were taken on JEM-H-7650. X-Ray photoelectron
spectroscopy (XPS) was recorded on a PHI-5702 instrument
and the C1S line at 292.1 eV was used as the binding energy
reference. Magnetic properties were measured at room tem-
perature using a vibrating sample magnetometer (VSM) at a
maximum field of 1 T. Thermogravimetric analysis (TGA) was
performed on TGA/SDTA851e under air atmosphere from 25
to 800 °C, with a heating rate of 20 °C/min and an air flowing
rate of 30 mL/min.
3 Results and Discussion
The preparation of the catalyst was schematically described
in Scheme 1. The content of Pd in the support catalyst was
found to be 0.105 mmol g-1 (determined by ICP-AES
analysis). First, Fe3O4@PEG–amine functionalized imida-
zolium ionic liquid was synthesized. Second, Pd(0)
nanoparticles through the reduction of Na2PdCl4 by hy-
drazine. The functionalized PEG-supported palladium
complex is dark solid and is readily dispersible in water to
form stable and monodisperse solution. This result gives a
direct evidence of PEG coating, because bare Fe3O4
magnetic nanoparticles are completely insoluble in water.
In addition, the structure of prepared materials was char-
acterized by various techniques including FT-IR, TEM,
VSM, XPS, XRD and TGA.
2.2 Synthesis of Fe3O4@PEG–Amine
Functionalized Imidazolium Ionic Liquid
(Fe3O4@PEG–AIm–NH2–IL)
We measured the Fourier transform infrared (FT-IR)
spectra of bare Fe3O4, PEG–AIm–NH2–IL and Fe3O4@
PEG–AIm–NH2–IL–Pd. In Fig. 1a, the band at 575 cm-1
was assigned to the vibration of the Fe–O bond, which
demonstrated the existence of Fe3O4 components. The
spectrum of free PEG–AIm–NH2–IL was also shown in
Fig. s1 (curve b) in Electronic Supplementary Material.
The observation of a band at around at 3,420 cm-1 indi-
cated the presence of the –NH2 group due to NH-stretching
vibration peak. Further, a diagnostic bending vibration
peak appeared at 1,633 cm-1. Compared with the curve a
and b, the characteristic adsorption band of Fe3O4 and the –
NH2 group at 575 cm-1, 3,420 cm-1 and 1,633 cm-1 were
all found in the spectra of Fe3O4@PEG–AIm–NH2–IL–Pd
(Fig. 1c). These results proved the existence of magnetic
Fe3O4 nanoparticles and PEG–AIm–NH2–IL components
in the new solid material.
500 mg Fe3O4 magnetic nanoparticles was dispersed in
50 mL deionized water by sonication for 30 min, and then
1.0 g PEG4000 bridged amine functionalized imidazolium
IL was added into it to form a stable black solution. The
mixture was stirred at 50 °C for 24 h under an argon at-
mosphere. After 24 h, the nanoparticles were separated
using an external magnet and washed repeatedly with
deionized water, and then dried in a vacuum at 40 °C for
24 h.
2.3 Synthesis of the Supported Catalyst
(Fe3O4@PEG–AIm–NH2–IL–Pd)
The Fe3O4@PEG–AIm–NH2–IL was added to aqueous
Na2PdCl4 solution with mechanical stirring, followed by
123

X. Liu et al.
Scheme 1 Preparation of Fe3O4@PEG–AIm–NH2–IL–Pd catalyst
Fig. 1 TEM image of fresh Fe3O4@PEG–AIm–NH2–IL–Pd (left) and the catalyst after 5th run of recycling (right)
The TEM micrograph for magnetite coated particles was
reported in Fig. 1 (left). It is possible to observe that the
single particles are nanometric size range with an average
size of about 20–30 nm, they are well dispersed and they
do not show strong tendency of microscopic aggregation.
The magnetic properties of the bare Fe3O4 NPs and
Fe3O4@PEG–AIm–NH2–IL–Pd were investigated using a
vibrating sample magnetometer at room temperature. As
illustrated in Fig. 2, the magnetization curves of the pre-
pared materials exhibited no hysteresis loop which
demonstrated its superparamagnetic characteristics. More-
over, the strong magnetic properties of the prepared cata-
lyst were revealed by complete and easy attraction using an
external magnet. The saturation magnetic moments
obtained was 60 emu g-1 for the bare Fe3O4 NPs, as shown
in the Fig. 3. As expected, after functionalized by PEG–
AIm–NH2–IL, it was reduced to 35 emu g-1, which may be
attributed to the surface disorder or spin canting at surface
of NPs. A direct result of this effect was that it took longer
time to separate Fe3O4@PEG–AIm–NH2–IL–Pd than bare
Fe3O4 NPs from particle solution.
The high angle XRD patterns of the Fe3O4 NPs and
Fe3O4@PEG–AIm–NH2–IL–Pd NPs were shown in Fig. 3.
It could be seen that the strong characteristic diffraction
peaks at 2h of 30.09o, 35.44o, 43.07o, 53.43o, 56.96o, and
62.55o corresponding to the diffraction of (220), (311),
(400), (422), (511), and (440) of the Fe3O4 were found in
both samples. This result meant that the core/shell Fe3O4/
123

Pd Nanoparticles Immobilized on Fe3O4 @ Poly(ethylene glycol) Bridged Amine Functionalized…
PEG–AIm–NH2–IL microspheres had been synthesized
successfully without damaging the crystal structure of Fe3O4
core. Moreover, apart from the original peaks, the appear-
ance of the new peaks at 2h of 40.01o, 46.53o, and 67.93o,
which corresponding to (111), (200), and (220) crystalline
planes of Pd was observed in the spectrum (Fig. 4b).
X-ray photoelectron spectroscopy (XPS) was recorded
on a PHI-5702 instrument and the C1s line at 292.1 eV was
used as the binding energy reference. The XPS elemental
survey scans of the surface of Fe3O4@PEG–AIm–NH2–
IL–Pd are showed in Fig. 4. Peaks corresponding to iron,
fluorin, oxygen, nitrogen, palladium, carbon were cleared
observed. To ascertain the oxidation state of the Pd, XPS
studies were carried out. In Fig. 4b, the Pd binding energy
of Fe3O4@PEG–AIm–NH2–IL–Pd exhibited two strong
peaks centered at 340.5 and 335.2 eV, which were assigned
to Pd 3d3/2 and Pd 3d5/2, respectively. These values agreed
with the Pd(0) binding energy of Pd NPs.
Fig. 2 Magnetization curves for Fe3O4 and Fe3O4@PEG–AIm–
NH2–IL–Pd
In addition, in order to further investigate the stability of
the catalyst during the catalytic process, TGA-DSC mea-
surements for the freshly prepared Fe3O4@PEG–AIm–
NH2–IL–Pd had been carried out. Fig. s2 illustrated the
TGA-DSC curves of the novel catalyst indicating the
weight change with temperature (T) (see Supporting in-
formation). With increasing T, the organic PEG–AIm–
NH2–IL component was completely decomposed to escape
as a gas at 244 °C. The result showed that the prepared
composite had high thermal stability.
We next focused our attention to explore catalytic ap-
plications of the novel magnetic-supported catalyst for
Heck reaction in water. Initially, the reaction conditions for
the Heck reaction were optimized by performing the model
reaction between iodobenzene and styrene. The results
from different reaction conditions are summarized in
Table 1. The effects of the additives, such as organic and
inorganic bases, were investigated in the coupling reaction.
Fig. 3 XRD patterns of Fe3O4 NPs (a) and Fe3O4@PEG–AIm–NH2–
IL–Pd (b)
Fig. 4 a XPS spectrum of the elemental survey scan of Fe3O4@PEG–AIm–NH2–IL–Pd and b XPS spectrum of the Fe3O4@PEG–AIm–NH2–
IL–Pd showing Pd 3d5/2 and Pd 3d3/2 binding energies
123

X. Liu et al.
Table 1 Heck reaction of iodobenzene with styrene catalyzed by Fe3O4@PEG–AIm–IL–Pd
Entry
Base
Ta (h)
Pd (mol%)
Yieldb(%)
1
2
Et3N
5
5
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.01
0.03
0.10
56
69
99
72
76
64
60
62
90
99
Pyridine
K2CO3
K3PO4
NaHCO3
NaOAc
KF
3
5
4
5
5
5
6
5
7
5
8
K2CO3
K2CO3
K2CO3
24
6
9
10
3
Reaction conditions: iodobenzene (1.0 mmol), styrene (2.0 mmol), base (2.0 mmol), H2O3 mL and Fe3O4@PEG–AIm–IL–Pd as catalyst
a
The reaction was monitored by GC
b
GC Yield
Table 2 Heck Reaction of Aryl Halides with terminal olefins catalyzed by Fe3O4@PEG–AIm–NH2–IL–Pd under the optimal conditions
Entry
R1
X
R2
Timea/h
Yieldb/ %
1
4-OMe
4-COMe
4-Br
I
Ph
7
5
95 (98/2)
98 (99/1)
93 (96/4)
94 (95/5)
91 (96/4)
97 (98/2)
90 (93/7)
90 (91/9)
95 (94/6)
90
2
I
Ph
3
I
Ph
5
4
4-Cl
I
Ph
5
5
4-Cl
I
4-MeCO–C6H4–
4-OMe–C6H4–
3-OMe–C6H4–
Ph
5
6
4-Cl
I
5
7
4-Cl
I
5
8
2-Me
I
5
9
4-Me
I
Ph
5
10
11
12
13
14
15
H
I
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
5
4-Me
I
5
91
4-COMe
2-Iodothiophene
4-NO2
4-NO2
I
5
95
I
5
85
76c
Br
Cl
10
24
Trace
Reaction condition: ArX (1.0 mmol), terminal olefins (2.0 eq), K2CO3(2.0 eq), catalyst Fe3O4@PEG–AIm–NH2–IL–Pd (10 mg 0.05 mol % Pd)
and H2O (3 mL)
a
The reaction was monitored by GC
b
GC Yield(the ratio of trans and cis product)
c
The reaction temperature: 100 °C, time: 10 h and Pd (0.1 mol %)
123

Pd Nanoparticles Immobilized on Fe3O4 @ Poly(ethylene glycol) Bridged Amine Functionalized…
Table 3 Recycle efficiency of Fe3O4@PEG–AIm–NH2–IL–Pd in Heck coupling between iodobenzene and styrene
Run
1
2
3
4
5
Yielda (%)
99
97
98
95
94
Reaction conditions: iodobenzene (1.0 mmol), styrene (2.0 mmol), K2CO3 (2.0 mmol), H2O 3 mL and Fe3O4@PEG–AIm–IL–Pd as catalyst
a
GC Yield
Table 4 Comparison of other amine functionalized Fe3O4 supported palladium catalyst on Heck reaction of iodobenzene with styrene in pure
water
Entry
Catalyst
Solvent
Yielda (%)
Ref
1
2
3
Fe3O4@NH2–Pd
H2O
H2O
H2O
55
80
99
28
Fe3O4@SiO2–iminophosphine–Pd
Fe3O4@PEG–AIm–NH2–IL–Pd
29
This work
Reaction conditions as exemplified in the experimental procedure
a
GC yield
The use of K2CO3 resulted in excellent yield of the desired
product (Table 1, entry 3). Further optimization suggested
that the amount of catalyst had a strong effect on this
process. The best performance was obtained when
0.05 mol % Pd was used. Increasing the catalyst loading
did not have a significant beneficial effect on catalytic
performance during the reaction. On the basis of the above
experiments, the optimized reaction conditions are use of
Fe3O4@PEG–AIm–NH2–IL–Pd (0.05 mol % Pd) as a
catalyst in water at 80 °C with 2.0 equiv K2CO3 for 5 h.
With the optimized reaction conditions in hand, the
scope of the Heck reaction of aryl halides with substituted
olefins was investigated and the results were present in
Table 2. In general, the aryl iodides with electron donating
or electron withdrawing groups both afforded the corre-
sponding products in excellent yields under the same
condition. Aryl iodides with electron-withdrawing groups
afforded the corresponding products in excellent yields
(Table 2, entry 2) Meanwhile, aryl iodides with electron-
donating groups required prolonged reaction time to obtain
excellent yields (Table 2, entry 1). It should also be noted
that aryl iodides can be selectively coupled in the presence
of aryl bromides. When the substrate contains iodo and
bromo substituents, styrene coupled readily only with the
iodo side (Table 2, entry 3). Obviously, electron-rich ter-
minal alkenes provided the desired products in higher
yields than electron-poor terminal alkenes did (Table 2,
entries 5–6). The aryl iodides with substituted groups in
ortho and meta positions gave lower yields in comparison
with para-isomers due to steric hindrance (Table 2, entries
8–9). This method was also successful with ethyl acrylate
to afford the corresponding olefin in excellent yields
(Table 2, entries 10–13). In addition, iodide bearing hete-
rocyclic aromatic substituent was also found to afford the
desired product in 85 % yield (Table 2, entry 13).
However, the coupling of aryl bromide bearing strong
electron-withdrawing groups, such as 4-bromonitroben-
zene, resulted in lower yield even reacting at elevated
temperature (100 °C) for manifold reaction time (10 h)
with double loading of Pd (0.1 mol %) (Table 2, entry 14).
Unfortunately, as for the aryl chloride, even bearing strong
electron-withdrawing group such as 4-chloronitrobenzene,
only a trace amount of product could be detected (Table 2,
entry 15).
The recycling and recovery of the supported catalysts
are a very important issue from both the practical and en-
vironment points of view. Since the primary target of this
study was to develop a recoverable catalyst in aqueous
reaction medium, we further explored the reusability of the
catalyst in the model reaction between iodobenzene and
styrene under the conditions descried in Table 2. When the
reaction was finished, simply by applying an external
magnet to the reaction vessel a separation of the catalyst
was achieved and the resulting clear supernatant could be
decanted, which minimizes the loss of catalyst during
separation. As shown in Table 3, the catalysts could be
reused for at least five times without significant loss of
activity. This observation indicates that the amounts of the
leaching of Pd into the reaction mixture must not be sig-
nificant; this has also been quantitatively supported by ICP
analysis. The analysis showed that after five recycling runs
of the catalyst, only 0.9 wt% of Pd was leached. All these
123

X. Liu et al.
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In the study, A palladium-based catalyst supported on
novel Fe3O4@ Poly(ethylene glycol) bridged amine func-
tionalized imidazolium ionic liquid (Fe3O4@PEG–AIm–
NH2–IL) was successfully prepared by a facile method,
which shown high activity towards Heck coupling reaction
in pure water. The novel catalyst could be recovered in a
facile manner from the reaction mixture and a good yield
was obtained even after the catalyst was reused five times.
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